Alkaline coating for sheet steel



Patented Mar. 14, 1939 UNITED STATES PATENT OFFICE ALKALINE COATING FOR SHEET STEEL Weston Morrill, Pittsfield, Mass., assignor to General Electric Company, a corporation of New York No Drawing.

watt lossesnwhen the steel is employed as core material in electrical apparatus such as transformers and the like. It has been customary therefore to remove this scale, as far as possible, by means of an acid pickling process which is applied to the steel after it has been annealed.

It is one of the objects of the present invention to provide a processfor treating magnetic sheet material and particularly silicon steel whereby the usual pickling process, previously employed in the manufacture of such steel, may be omitted without causing any increase in the total watt losses which may be obtained with such material.

It is a further .object of the invention to prevent excessive scaling of annealed silicon steel and to provide means whereby thin rolled sheets may be easily separated from one another after the annealing process. V

In carrying out the present invention, I apply .a solution of sodium aluminate to rolled silicon steel sheets. The solution is preferably a to 12% solution in water and may be applied to the steel, by the, type of rolls generally employed for the application of core plate enamel. The solution is poured directly onto the rolls and evenly applied thereby to the steel.

The coated steel is dried in any suitable atmosphere either oxidizing, reducing or neutral, then placed in packs and annealed. Any ordinary an- 0 nealing cycle for silicon steel may be employed.'

Generally, however, I prefer to anneal the'packs for about 6 to 8 hours at a temperature in the neighborhood of 850 C. to about 875 C. When annealing temperatures above 1000 C. are employed the coating is particularly beneficial in keeping down the pufiy silicon steel scale generally formed at that temperature, The annealing atmosphere may be oxidizing, reducing or neutral as desired.

0 If desired, the coated sheets may be normalized in the usual way by heating single or spaced sheets quickly, i. e. in about ten minutes, to a temperature in the neighborhood of 1000 C. then quickly cooling.

The application of the sodium aluminate to the Application June .9, 1934, Serial No. 729,918

steel not only permits ready separation of the n annealed sheets but prevents excessive scaling. Moreover, the quantity of scale obtained by the present process is far less than that obtained by annealing uncoated steel.

Any scale which is 5 developed on the coated sheets during the annealing process is not removed, as in the ordinary pickling process, but allowed to remain on the steel.

When silicon steel sheetscoated with a 10 to 10 12% water solution of sodium aluminate are annealed, as indicated above, and employed as the laminated core of a transformer, the total watt losses for such material at high flux densities such I as 10,000 to 15,000 B' are from about 1 to 5 lower than the total watt losses which may be obtained with uncoated annealed sheets. The improved watt losses, while noticeable at flux "densities of about 10,000 B, are particularly evident as the flux densities increase above that 20 point.

The total watt losses which are obtained by the use of my improved process are substantially the same as those which may be obtained when the sheet steel is annealed and then pickled. The present process, however, is far less'expensive than-that employing the pickling step. Further.- more, the coated annealed sheets in the pack may be separated more easily than the uncoated sheets. The present process is not limited in its application to any particular silicon steel but may be employed to advantage with all commercial grades of silicon steel in which the silicon content may vary, for example, from about 1 to about 7%.

Although I prefer to employ sodium aluminate as the coating material, my invention is not limited to the use of that material. Other aluminates, alkaline materials, or materials having an alkaline reaction such as potassium aluminate, sodi- 40 um hydroxide, sodium carbonate, sodium phosphate and equivalent materials may be employed if desired. In addition to the above, there are some materials which, when applied in a water solution to the silicon steel sheets, may have a neutral or even an acid reaction. However, if under the influence of the heat of the anneal, the material decomposes forming an alkaline reaction product, the watt losses of the steel will be improved. An example of =this type of material is potassium fluoride whose water solution is neutral. and which will decompose during the anneal, the fluorine and potassium reacting with silica in the steel scale to form volatile silicon tetrafluoride, potassium oxide and potassium silicate, both latter products being alkaline materials.

Many different coating materials may be employed with satisfactory results. However, I prefer to employ sodium aluminate, particularly since it has a high melting point. This is a distinct advantage in preventing sticking of the sheets during the annealing process.

Although I prefer to employ a. 10 to 12% sodium aluminate solution, other concentrations of the coating solution may be employed if desired. For example, I have obtained good results with a 2% and even more dilute solutions. However, if the concentration of the solution is increased to about 50% the coating has a tendency to cake and to cause sticking of the steel sheets during anneal.

What I claim as new and desire to secure by and thereafter drying the coated steel and annealing it, said coating having an alkaline reaction.

2. The method for improving the total watt losses in unpickled silicon steel which comprises applying an alkaline solution to the unpickled steel and thereafter drying the coated steel and annealing it.

3. The process oftreating rolled sheet like silicon steel which comprises applying an aluminate to the rolled steel and annealing the coated steel.

4. The process of treating rolled sheet like silicon steel which comprises applying sodium aluminate to the rolled steel and annealing the coated steel.

5. A finished, rolled silicon steel sheet having a coating of sodium aluminate thereon.

6. A finished rolled silicon steel sheet having an aluminate coating thereon.

7. The method for treating unpickled silicon WESTON M RRILL. 

